A protein kinase is an enzyme which catalyzes phosphorylation of hydroxyl groups on tyrosine, serine and threonine residues of proteins. It plays an important role in signal transduction of growth factors involved in growth, differentiation and proliferation of cells.
To maintain homeostasis, it is necessary to keep good balance in turning on and off of the signal transduction system. However, mutation or overexpression of specific protein kinases disrupts the signal transduction system in normal cells and causes various diseases including cancers, inflammations, metabolic diseases, brain diseases, etc. Examples of the typical protein kinases that lead to diseases caused by abnormal cell growth are Raf, KDR, Fms, Tie2, SAPK2a, Ret, Abl, Abl (T315I), ALK, Aurora A, Bmx, CDK/cyclinE, Kit, Src, EGFR, EphA1, FGFR3, Flt3, Fms, IGF-1R, IKKb, IR, Itk, JAK2, KDR, Met, mTOR, PDGFRa, Plk1, Ret, Syk, Tie2, TrtB, etc.
It is estimated that there are 518 different kinds of protein kinase genes in humans constituting about 1.7% of the entire human genes [Manning et al., Science, 2002, 298, 1912]. Human protein kinases are largely divided into tyrosine-specific protein kinases and serine/threonine-specific protein kinase. The tyrosine-specific protein kinases may be divided into 58 receptor tyrosine kinases, which are grouped into 20 subfamilies, and 32 cytoplasmic/non-receptor tyrosine kinases, which are grouped into 10 subfamilies. The receptor tyrosine kinase has an extracellular domain capable of binding to a growth factor and a cytoplasmic active site that can phosphorylate the tyrosine residue. When a growth factor binds to the extracellular growth factor receptor site of the receptor tyrosine kinase, the receptor tyrosine kinase forms a dimer and the tyrosine residues in cytoplasm are autophosphorylated. Then, the downstream proteins are sequentially phosphorylated, and as the signal transduction proceeds in the nucleus, the transcription factors that induce cancer are overexpressed in the end.
Focal adhesion kinase (FAK) is a 125 kD tyrosine-specific protein kinase present in cytoplasm. FAK plays a critical role in migration, proliferation and survival of cells by regulating the signal transduction system of integrin and growth factors. FAK protein and FAK mRNA were found to be overexpressed/activated in various cancer cells, including squamous cell carcinoma, invasive rectal cancer/breast cancer, metastatic prostate cancer, melanoma and glioma. Novartis' FAK inhibitor TAE226 [Cancer Invest. 2008, 26(2), 145] was shown effective for the treatment of breast cancer through three different kinds of animal models (HeyA8, SKOV3ip1 and HeyA8-MDR) [Cancer Res. 2007, 67(22), 10976)]. Further, Pfizer's FAK inhibitor PF-573,228 [Proc. Am. Assoc. Cancer Res., 2006, 47, Abst. 5072] is under successful clinical trial. It was shown effective for the treatment of prostate cancer (PC-3M), breast cancer (BT474), pancreatic cancer (BxPc3), lung cancer (H460) and brain cancer (U87MG) in animal models. In addition, a concurrent administration of FAK inhibitor (TAE226) and docetaxel showed an excellent efficiency (85-97% tumor reduction, P values<0.01) in an animal model [Cancer Res. 2007, 67(22), 10976].
FAK is involved in the signaling of integrin. When integrin receptors duster in response to various stimulations from outside, the cytoplasmic domain (cytoplasmic tail) of integrin binds to the cytoskeleton and signaling proteins. The FERM (F for 4.1 protein, E for ezrin, R for radixin and M for moesin) domain and the focal adhesion targeting (FAT) domain of FAK independently bind to the cytoplasmic domain of integrin and allow the FAK to be located at the focal adhesion site. The FAKs clustered close to the focal adhesion site are activated via intramolecular or intermolecular phosphorylation of the Y397 residue. Then, the SH2 domain of Src kinase binds to the phosphorylated Y397 residue of FAK to form an FAK/Src complex. The Src kinase bound to FAK further phosphorylates other tyrosine residues (Y407, Y576/577, Y861 and Y925) of FAK. Also, the FAK/Src complex binds to various signaling proteins (P130Cas, Grb2, PI3K and Grb7) and mediates phosphorylation. In normal cells, the signal transduction through FAK is mediated under strict regulation. However, in tumorized cells, FAK is overexpressed and activated thereby exhibiting various features of malignant tumors. FAK facilitates proliferation of cancer cells, increases invasion, and migration of cancer cells. Further, FAK is also known to suppress cancer cell apoptosis and increase angiogenesis.
FAK is a protein targeted by many growth factor receptors including epidermal growth factor receptor (EGFR) and platelet-derived growth factor receptor (PDGFR), as well as integrin. Overexpression of the receptors or expression of activated receptors converts normal cells into tumor cells. Thus, FAK is an important kinase involved in tumor-related signal transduction of the receptors. It has been reported that the N-terminal FERM domain of FAK binds to EGFR and the C-terminal domain of FAK is involved in the cell migration mediated by epidermal growth factor (EGF). That is, FAK recognizes the signal from the EGFR receptor through the N-terminal FERM domain and recognizes the signal from the integrin through the C-terminal FAT domain, thereby integrating signals from the outside of the cell.
Apoptosis may be induced by inhibiting FAK in various manners. Cell survival mediated by FAK is mainly conducted by phosphoinositide 3-kinase (PI 3-kinase). The phosphorylated Y397 site of FAK binds to PI 3-kinase and synthesizes PI(3,4,5)P3 and PI(3,4)P2 as second messengers, which move protein kinase B (PKB, also called AKT) to the cell membrane so that it can be phosphorylated by 3′-phosphoinositide-dependent kinase (PDK). Thus activated PKB deactivates apoptotic proteins (e.g., p21WAF, FKHR, Bad and GSK-3), and thereby inhibits apoptosis. Another signal for survival is the binding of the SH3 domain of p130Cas to the proline-rich motif of FAK, whereby phosphorylation of the tyrosine residues of p130Cas is induced by FAK/Src and Ras is activated.
The role of FAK in the cell cycle is explained as follows. If the Y925 site is phosphorylated, FAK binds to growth factor receptor-bound protein 2 (Grb2) thereby activating the Ras/Erk pathway. Overexpression of FAK facilitates G1 to S phase transition, and expression of FAK related non-kinase (FRNK), an inhibitor of FAK, inhibits the expression of cyclin D1 and induces the expression of the CDK inhibitor p21, thereby delaying the progress of the cell cycle. However, overexpression of cyclin D1 rescues the cells from the cell cycle arrest by FRNK.
The only subtype of FAK, proline-rich tyrosine kinase 2 (PYK2), is the most highly distributed in nerve cells. Recently, it was reported as a useful molecular target in the development of anticancer drugs for small-cell lung cancer [Oncogene. 2008, 27(12), 1737], prostate cancer [Oncogene. 2007, 26(54), 7552], liver cell carcinoma [Br. J. Cancer. 2007, 97(1), 50] and glioma [Neoplasia. 2005, 7(5), 435].
FAK comprises four domains: 1) The FERM (band 4.1 protein, ezrin, radixin, moesin) domain is an amino-terminal domain that interacts with integrin receptor, platelet-derived growth factor receptor (PDGFR), epidermal growth factor receptor (EGFR), etc. and inhibits kinase activity through direct interaction with the kinase domain; 2) the kinase domain; 3) three proline-rich (PR) regions; and 4) the focal adhesion targeting (FAT) domain positioned at the carboxyl-terminal interacts with paxillin, talin, p190RhoGEF, RhoA-specific GDP/GTP exchange factor, etc. The alternative splicing product of FAK, FAK-related non-kinase domain (FRNK), consists of PR1, PR2 and FAT domains and acts as an antagonistic regulatory factor of FAK.
For the activation of FAK, it is essential to have autophosphorylation of Y397 located at the junction of the FERM and kinase domains. Src kinase binds to the phosphorylated Y397 and sequentially phosphorylates Y576 and 577. When Y925 is phosphorylated in the end, the signal transduction of FAK is turned on through Grb2. The FAK inhibitors currently under development are shown to inhibit the autophosphorylation of Y397 by targeting the ATP binding site of the kinase domain. The extent of the inhibition of Y397 autophosphorylation is an important biomarker in the efficiency test using an animal model.
The progress that has been made in the development of low molecular weight FAK inhibitors is as follows. Of the 26 lead compounds that have been proposed for the FAK inhibitors, only the Pfizer's PF-562271 is under clinical trial phase I at present. PF-562271 is an ATP-competitive FAK inhibitor (IC50=1.5 nM) and a homologous Pyk2 inhibitor (13 nM). It inhibits autophosphorylation at the FAK Y397 site in fibroblasts, epithelial cells and cancer cells. Further, it inhibits the migration of most cancer cells, but does not affect the growth of normal cells. No special toxicity has been observed and inhibition of tumor growth or tumor degeneration by 42-90% was observed in in vivo human tumor xenograft tests (25-100 mg/kg p.o.) for prostate cancer PC-3, breast cancer BT-474, colon LoVo, lung cancer NCI-H460, glioblastoma U-87 MG and pancreatic cancer BxPC-3 cells.
Vascular endothelial growth factor receptors (VEGFRs) are receptor tyrosine kinases (RTKs) and important regulatory factors of angiogenesis. They are involved in the formation of blood vessels and lymphatic vessels and in homeostasis, and exert important effects on nerve cell. Vascular endothelial growth factor (VEGF) is produced mostly by vascular endothelial cells, hematopoietic cells and stromal cells under a hypoxic condition or by stimulations from growth factors such as TGF, interleukin and PDGF. VEGF binds to VEGFR-1, -2 and -3. Each VEGF isoform binds to a specific receptor, thereby inducing the formation of a receptor homozygote or heterozygote, and activates each signal transduction system. The signal specificity of VEGFR is further fine-tuned by co-receptors such as neuropilin, heparan sulfate, integrin, cadherin, or the like.
The biological function of VEGF is mediated by type III RTK, VEGFR-1 (Flt-1), VEGFR-2 (KDR/Flk-1) and VEGFR-3 (Flt-4). VEGFR is closely related to Fms, Kit and PDGFR. Each VEGF binds to specific receptors. VEGF-A binds to VEGFR-1, -2 and receptor zygote, whereas VEGF-C binds to VEGF-2, -3. PIGF and VEGF-B interact exclusively with VEGFR-1, and VEGF-E interacts only with VEGFR-2. VEGF-F interacts with VEGFR-1 or -2. Whereas VEGF-A, -B and PIGF are preferentially required for the formation of blood vessels, VEGF-C and -D are essential in the formation of lymphatic vessels. Angiogenesis is essential in the proliferation and transition of tumors, since it supplies nutrients and oxygen to the tumors and provides channels for transition to cancer cells. Normally, angiogenesis is balanced by angiogenesis promoters and angiogenesis inhibitors. If the balance is broken, as in cancer cells, the growth factor that affects the vascular endothelial cells most, i.e., VEGF, activates its receptor, VEGFR. At present, various researches are under way on the inhibitors that inhibit the receptor tyrosine kinase of VEGF using low molecular weight synthetic substances, which are advantageous in that they are applicable also to solid tumors and have fewer side effects because they inhibit angiogenesis in the cancer cells only.
Tie2 is a kind of receptor tyrosine kinase and is deeply involved with angiogenesis and vasculature. The domain structure of Tie2 is very highly conserved in all vertebrates [Lyons et al., 1998]. The ligand of Tie2 is angiopoietin (Ang). Ang2 does not induce autophosphorylation of Tie2, but interferes with the activation of Tie2 by Ang1. In endothelial cells, the activation of Tie2 by Ang2 induces activation of PI3K-Akt [Jones et al., 1999]. In the mitogen-activated protein kinase (MAPK) signal transduction pathway, which is the main signal transduction system of Tie2, the adaptor protein GRB2 and the protein tyrosine phosphatase SHP2 play a key role in dimerization of the Tie2 receptor tyrosine kinase through autophosphorylation. Ang/Tie2 and the VEGF signal transduction pathway are important in angiogenesis of cancer cells. Tie2 is expressed in vascular endothelial cells. Especially, the expression increases remarkably at the site invaded by cancer cell. Overexpression of Tie2 was observed in breast cancer [Peters et al., 1998] and also in uterine cancer, liver cancer and brain cancer.
Several compounds with the thieno[3,2-d]pyrimidine structure have been synthesized. However, the substituted thieno[3,2-d]pyrimidine compound of the present invention with specific substituents at the 2-, 4- and 7-positions of thieno[3,2-d]pyrimidine is a novel compound not disclosed in any literature. Moreover, the inhibition activity against various protein kinases or the possibility of the substituted thieno[3,2-d]pyrimidine compound with the specific substituents at the 2-, 4- and 7-positions has not been predicted in any literature.